In biology, evolution refers to the processes that cause some inherited traits to become more common relative to others over time. Natural selection, the most important of these, works because some traits or collections of traits will, on average, increase reproductive success, either by protecting the organism from dying before it can reproduce, or by enhancing his reproductive success directly, such as the peacock's plumage. As genes are passed on by reproduction, those that increase reproductive success are more likely to be passed on, relative to neutral or relatively unfavourable traits, and so, generation to generation, the number of organisms with these traits will tend to increase, unless conditions change so as to make them no longer favourable. Other, less important mechanisms of change include gentic drift (random changes in frequency of traits, most important when the traits are, at that time, reproductively neutral), and the founder effect, in which, if a small group of organisms become isolated from the main population, that isolated population will, even many generations on, tend to have a higher frequency of any rare variations in the founders as compared to the original, larger population.

Over time, the accumulation of many of these inherited changes can result in the development of new species from existing ones, a process called speciation. [Explain major routes of speciation: geographical differences, isolation, divergence into seperate niches, etc]

[Simple evidence, probably the Universal genetic code] and other discoveries have shown that this mechanism can be traced back to a single common ancestor, from which all living organisms on Earth descended as a result of cumulative evolutionary change over billions of years, making evolution responsible for the vast diversity of life on Earth, including the many extinct species found in the fossil record.